A NAND flash memory is known as a kind of nonvolatile semiconductor memory. Each memory cell in the NAND flash memory has a stacked gate structure with a first insulating film, a charge storage layer, a second insulating film, and a control gate electrode stacked on a semiconductor substrate in order. The first insulating film is called a “tunnel insulating film”. The second insulating film is called an “inter-gate insulating film” if the charge storage layer is a floating gate electrode comprising polycrystalline silicon and is called a “block insulating film” if the charge storage layer comprises an insulator. Silicon oxide or oxynitride has hitherto been used as the first insulating film and the second insulating film. However, with increasing miniaturization of memory cells, insulating films with increased dielectric constants (high-k film) have been examined.
The high-k film enables a reduction in effective oxide thickness (EOT), with the physical film thickness thereof maintained. Thus, the high-k film allows the memory cells to be miniaturized, while enabling a reduction in the operation voltage of memory elements. Moreover, the high-k film is expected to suppress leakage current, which mainly comprises FN (fowler nordfeim) tunnel current, in the high electric field region. However, compared to the silicon oxide film or the oxynitride film, there is a tendency that oxygen defects are likely to be formed in the high-k film because the high-k film is often formed of a material based mainly on ion bonding instead of covalent bonding. The oxygen defects often form an energy level in a band gap. The energy level may prevent a leakage current in a low electric field region from being suppressed, thus degrading data retention characteristics. Hence, simultaneously improving the leakage current characteristics of both the high and low electric field regions is difficult.
On the other hand, a stacked structure with a silicon oxide film or an oxynitride film and a high-k film has been proposed in order to suppress the leakage current in the low current field region. However, silicon oxide or oxynitride has a small dielectric constant, thus reducing the series capacity of the high-k film and the silicon oxide or oxynitride. This disadvantageously makes a reduction in EOT difficult, resulting in a failure to decrease the operation voltage.